Petroleum refiners often produce desirable products, such as gasoline and middle distillates by catalytically hydrocracking high boiling hydrocarbons into product hydrocarbons of lower average molecular weight and boiling point. Hydrocracking is generally accomplished by contacting, in an appropriate reactor vessel, a gas oil or other hydrocarbon feedstock with a suitable hydrocracking catalyst under appropriate conditions. These conditions include elevated temperature and elevated pressure and the presence of hydrogen, such that a hydrocarbon product is obtained containing a substantial portion of a desired product boiling in the range of 85° C. to 215° C. or middle distillate boiling in the range of 150° C. to 425° C.
Usually, hydrocracking is practised in a single reaction vessel or several reaction vessels in series utilising a single catalyst. In such a scenario, the catalyst not only hydrocracks the hydrocarbon feedstock into lower boiling products but simultaneously or sequentially converts the organonitrogen- and organosulphur-containing compounds into ammonia and hydrogen sulphide. Some isomerisation of normal or near normal paraffins can also take place simultaneously. Such an operation is broadly termed a single stage operation.
Hydrocracking can also be performed in conjunction with hydrotreating, usually by a method referred to as “integral operation”. In this process, the hydrocarbon feedstock, usually gas oil containing a substantial proportion of components boiling above a desired end point as for example 215° C., is introduced into a catalytic hydrotreating zone in the presence of a suitable catalyst. This catalyst can be a zeolite or sieve-free particulate catalyst comprising a Group VIII metal component and a Group VIB metal component on a porous, inorganic, refractory oxide catalyst support most often composed of alumina. Suitable process conditions include elevated temperature (e.g. 230° C. to 455° C.) and elevated pressure (e.g. 4 to 35 Mpa) and hydrogen as a reactant.
The organonitrogen components and the organosulphur components contained in the feedstock are converted to ammonia and hydrogen sulphide, respectively. Subsequently, the entire effluent is removed from the hydrotreating zone and treated in a hydrocracking zone maintained under suitable conditions of elevated temperature, pressure and hydrogen partial pressure. The hydrocracking zone contains a suitable hydrocracking catalyst, such that a substantial conversion of high boiling feed components to products components boiling below the desired end point is obtained. Usually, the hydrotreating and hydrocracking zones in integral operation are maintained in separate reactor vessels, but, on occasion, it may be advantageous to employ a single, downflow reactor vessel containing an upper bed of hydrotreating catalyst particles and lower bed of hydrocracking particles.
Examples of integral operation may be found in U.S. Pat. Nos. 3,132,087; 3,159,564; 3,655,551 and 4,040,944, all of which are herein incorporated by reference in their entireties. The unconverted product from the hydrocracking bed may or may not be recycled to either of the prior catalysts. Such an operation is also referred to as a single stage process.
When two catalysts in two separate vessels are used, it is often desirable to fractionate (or separate) the products of the first reactor (hydrotreating) so as to remove the produced ammonia, hydrogen sulphide and light hydrocarbons from the feed to the hydrocracking reactor. Examples of such processes are disclosed in U.S. Pat. Nos. 3,923,638 and 4,211,634 incorporated by reference. Such separation can also be made when two similar catalysts are used.
In some integral operation refining processes, and especially those designed to produce middle distillate from the heavier gas oils, a relatively high proportion of the product hydrocarbons obtained from integral operation will have a boiling point above the desired end point. For example, in the production of a middle distillate product boiling in the 180-390° C. range from a gas oil boiling entirely above 300° C., it may often be the case that as much as 30° C. to 60° C. percent by volume of the products obtained from integral operation boils above 390° C. To convert these high boiling components to hydrocarbon components boiling below 390° C., the petroleum refiner separates the 390° C. high boiling components from the other products obtained in integral operation, usually after first removing ammonia by a water washing operation, a hydrogen-containing recycle gas by high pressure separation and an H2S containing C1 to C3 low BTU gas by low pressure separation. This 390° C. boiling bottom fraction is then subjected to further hydrocracking, either by recycle to the hydrocracking reactor in single stage operation or by introduction into a second hydrocracking zone whereby yet more conversion to the desired 180-390° C. product takes place.
Further description of two-stage hydrocracking operations may be found in U.S. Pat. Nos. 4,429,053 and 4,857,169 herein incorporated by reference in their entireties. These patents provide process flow sheets for typical two-stage hydrocracking processes.
U.S. Pat. No. 4,875,991 discloses a two-zone hydrocracking process in which a feedstock is contacted with a first reaction zone catalyst comprising hydrogenation components essentially of a nickel compound and a tungsten compound deposited on a support consisting essentially of an alumina component and a crystalline molecular sieve component and contacting the effluent from first reaction zone in a second reactor zone with a catalyst comprising a hydrogenation component consisting essentially of a molybdenum component deposited on a support component consisting essentially of an alumina compound and a crystalline molecular sieve component.
An essential disclosure is the superiority of a molybdenum compound catalyst in zone 2. Furthermore, this patent does not teach the use of beta zeolite and in addition does not disclose the benefits of using two different catalysts containing two different molecular sieves in the different zones.
U.S. Pat. No. 4,851,109 discloses a two zone process using a large pore molecular sieve in the first zone and a beta zeolite in the second zone in which the product from the first zone is separated and only the unconverted first zone product is fed to the second hydrocracking zone.
U.S. Pat. No. 5,935,414 discloses a dual catalyst system specifically for a process designed to convert a wax containing feedstock containing a substantial portion of hydrocarbons boiling above 343° C. into middle distillate product with a reduced wax content, which comprises (a) contacting the feed in the presence of hydrogen with a catalyst containing a carrier, hydrogenation components selected from Group VIB and Group VIII and a large pore Y zeolite having a pore diameter in the range of 0.7 to 1.5 nm in a hydrocracking zone and (b) passing the entire effluent into a second zone containing a crystalline, intermediate pore size molecular sieve selected from the group of metallosilicates and silica-alumina phosphates and having a pore diameter in the range of 0.5 to 0.7 nm in a hydrodewaxing zone. Suitable zeolites for use in the first zone include X, Y, L, omega, beta, and their modifications. Suitable zeolites for use in the second zone include SAPO 11, 31, 34, 40, 41, ZSM 5, ZSM 11, −12, −23, −35 and −38, ZSM 5 being preferred. It is seen that zeolite beta is a suitable component for zone 1 but not for zone 2.
WO Patent Application No. 00/69993 discloses a dual catalyst system for hydrocracking heavy naphtha feedstocks into gasoline blending stocks and lighter compounds. According to the text, the feedstock maximum boiling point is about 240° C. It is disclosed that an unexpected increase in selectivity to liquid products is obtained when partial or complete aromatics saturation is achieved prior to hydrocracking. The dual catalyst system used is able to decouple the aromatics saturation and hydrocracking reactions. The feedstock is defined as being a straight run thermally or catalytically cracked naphtha typically boiling below 260° C. The product is usually gasoline blending stock. Compared to a single catalyst system, the yield of C1 to C4 is decreased and yield of C5-260° C. product increased.
The preferred aromatics saturation catalyst consists of a noble metal supported on a Y zeolite operating in a pressure range of 350-1200 psig, and conversions are typically greater than 30 vol %. A highly active zeolite beta with a molar silica-alumina ratio in the range of 10-200 is desirable for use as the hydrocracking catalyst in the process. In the preferred process, the aromatics saturation zone is operated at a lower temperature than the cracking zone.
U.S. Pat. No. 4,906,353 discloses dual mode hydrocracking conversion process. The feedstock is first treated under reforming conditions and then hydrocracked. The process is directed towards producing a relatively low yield of high octane hydrocrackate and a relatively high yield of C2-C4 hydrocarbons.
U.S. Pat. No. 5,831,139 discloses a process combination to upgrade heavy naphtha to aliphatic gasoline.
U.S. Pat. No. 5,364,514 discloses passing a feedstock into one or more hydrocracking zones to effect the decomposition of organic sulphur and nitrogen components. A portion of the product is passed to an aromatics saturation zone and subsequently to a hydrocracking zone wherein the products are separated into a top fraction and a bottom fraction. Part or all of the bottom fraction is recycled to the hydrocracking zone and/or the aromatics saturation zone.
U.S. Pat. No. 6,620,295 B2 discloses a hydrocracking catalyst containing modified Y and zeolite betas contained in the same particles.
Hydrocracking catalyst comprising Y-zeolites are well known in prior art. Examples of prior art of hydrocracking utilizing Y zeolite are given in U.S. Pat. Nos. 4,851,109, 4,875,991 and 4,401,556, which are incorporated by reference. Prior art disclosure of beta zeolite containing hydrocracking catalysts is exemplified by U.S. Pat. No. 3,923,641, U.S. Pat. Nos. 3,128,924, 5,284,573 and 4,612,108, which are incorporated by reference, further illustrate the use of hydrocracking catalysts containing beta zeolite.
None of these patents disclose the use of a first hydrocracking zone containing a large pore zeolite catalyst, preferably a Y-zeolite, followed by a second hydrocracking zone containing a catalyst comprising a beta zeolite, in which the total product of the first zone is passed to the second zone.
Although several types of commercial hydrocracking catalysts exist, which can be used effectively in single stage hydrocracking or in either the first, second or both stages of the above discussed two-stage hydrocracking processes, there is always a demand for catalysts with superior overall activity, selectivity and stability for producing gasoline and/or middle distillate via hydrocracking.
The general object of the present invention is directed towards a novel process employing two different hydrocracking catalysts in two different zones in which the product of the first zone is passed in total to the second zone.
The present invention is particularly directed towards hydrocarbon conversion catalysts, and hydrocarbon conversion processes employing such catalysts comprising a hydrogenation component(s) on a catalyst support comprising a zeolite component.